Hi there
Everything is logged on the servers at every step of the purchase process so you are not in danger of losing your platinum.
However sometimes delays can happen between your device and the Apple server confirming the purchase, these are most often internet related delays.
There are a few things you can try:
First stop is to have a look at this support page and follow the steps outlined:
http://celtic-heroes.com/support/suppor ... um-issues/
Make sure you are playing on the same device and log into the same account as you purchased the platinum on.
Try a different connection (e.g. if you are currently on wifi try 3G, or try a wifi connection in another place.
As mentioned above there's no danger of actually losing the platinum but it helps to try and nudge things along if there have been delays verifying the purchase with the above steps.
Please let me know how you get on.